Transmitting an optical signal (e.g., bi-directional data transmission) through a rotary hinge is not an easy task, since uninterrupted optical fibers cannot be used.
A similar problem can be also found in hinges limited in its turning feature, e.g., to a range +/−180° hinges. In this case, in principle, one can lead optical fibers through the hinge, but then the durability of the optical fibers will be limited.
Bi-directional optical signal transmission is preferable for many practical applications, e.g., in mobile phones, where high-speed data transfer in two directions is required, e.g., where a display and a camera are located in the upper part of the phone and the image handling chip(s) are located in the lower part.